Compressed air systems are widely used in both industrial and automotive applications. One major use of compressed air systems is to operate the braking systems of heavy duty vehicles. Since moisture in compressed air denigrates the system components, it has become customary to provide an air dryer to remove the moisture entrained in the compressed air used in compressed air systems. Common air dryer designs include a desiccant bed through which the compressed air is passed. The moisture is purged from the desiccant bed periodically by backflowing, at relatively Low pressure, a quantity of compressed air which has been segregated and which is then discharged to atmosphere.
Compressed air braking systems are used on large line haul trucks, which may operate for several hours without having a brake application effected. Accordingly, the compressor is on load during a relatively short period of time during vehicle operation and is unloaded during the majority of time that the vehicle is operated. The air requirements for this type of application are relatively low, and the desiccant in the air dryer will be open to atmosphere for far longer than the time required to purge the moisture from the desiccant. On the other hand, air dryers are also used on vehicles equipped with compressed air braking systems in which air consumption is relatively high, such as transit buses and other delivery vehicles. In these vehicles, the compressor is not unloaded for long enough at any one time to assure complete purging of the moisture from the desiccant in the air dryer. Accordingly, many of these high air consumption vehicles are equipped with continuous flow air drying subsystems, in which a pair of air dryers are used so that one of the air dryers is being purged while the other one is drying air. A timing control is used to switch between the air dryers. This type of air drying system is commonly referred to as a "continuous flow" air dryer; the air dryer system used on heavy duty line haul trucks with relatively low air consumption requirements is commonly referred to as "intermittent type" air drying systems.
In the so-called intermittent type systems, a small portion of the air is segregated in a purge volume and is used to backflow through the desiccant when the air compressor goes off load. The purge volume in which the purge air is stored is commonly made a part of the same housing containing the desiccant. However, this results in a much larger air dryer housing, thereby complicating installation requirements. Accordingly, some such systems are provided with a separate purge volume.